SSUSI and GUVI limb scans of thermospheric neutral density changes during a geomagnetic storm

The response of the upper atmosphere to the series of geomagnetic storms from 22-28 July 2004 were observed by two ultraviolet sensors: the first in a series of Special Sensor Ultraviolet Spectrographic Imagers (SSUSI) carried by DMSP satellites, and the Global Ultraviolet Imager (GUVI) experiment on the NASA TIMED satellite. The field of view of these nearly identical instruments sweeps from above the Earth limb downward across the Earth disk through satellite nadir to near the opposite limb. Far ultraviolet radiances provide comprehensive details about daytime O, N2, O2, and temperatures. While disk images provide a more direct measure of O/N2 column density ratio, limb profiles are fit with a model of the airglow mechanisms that iteratively estimates the most probable altitude profile of the number density of each species as well as the temperature profile. Successive scans create a two-dimensional altitude-latitude slice of the upper atmosphere that can be used to map and interpret global changes in the thermosphere. Because their orbits differ, SSUSI and GUVI can obtain dayglow measurements at different locations and times. While this means the combined data allow for enhanced spatial and temporal coverage, a reference standard such as the NRLMSISE-00 empirical neutral atmosphere model must be used for any intercomparisons. Neutral density enhancements derived from far ultraviolet airglow measured during the three storm bursts in this week are a factor of 1.5 higher than NRLMSISE-00 predictions and in general agreement with mass density values obtained from satellite drag calculations. The correlation is best below 400 km although qualitative agreement is seen at all altitudes.